Encoders are used as means for detecting positions of servo motors, for example. An example of the encoders primarily includes a detector and a signal processor.
The detector outputs an analog sine wave signal and an analog cosine wave signal in accordance with a position (a rotational angle and the like) of the motor. The signal processor converts the analog sine wave signal and the analog cosine wave signal output from the detector into digital signals using AD conversion and obtains the position of the motor. The signal processor uses a tracking technique as a technique to calculate the position of the motor. In the position calculation method by the tracking technique, the position of the motor is obtained by performing processing by the tracking technique once at AD conversion timing when the analog sine wave signal and the analog cosine wave signal from the detector are converted into the digital signals like an encoder as described in Japanese Patent Application Laid-open No. H4-328410 (FIG. 1).
In the encoder, however, the processing by the tracking technique by a signal processor does not follow an actual position of the motor when the position of the motor is rapidly changed, thereby causing a delay (a positional difference).